1. Field of the Invention
The present invention relates to a control circuit for instantaneous waveform control power converting apparatus for controlling an instantaneous output current, the power converting apparatus being, for example, a sine wave voltage outputting inverter including a motor driving inverter, a high power factor converter, an active filter, and an LC filter. More specifically, the present invention relates to a control circuit including of a plurality of switching elements which perform a plurality of switching operations within one cycle.
2. Description of the Related Art
Referring now to drawings, a description will be made of a conventional control circuit used in a power converting apparatus. FIG. 17 is a diagram for showing an arrangement of the conventional control circuit designed for a power converting apparatus, which is described in, for example, the Japanese publication entitled xe2x80x9cCURRENT CONTROL TYPE PWM INVERTER CAPABLE OF SUPPRESSING HIGHER HARMONIC AND OF REALIZING HIGH-SPEED CURRENT RESPONSExe2x80x9d of Japanese Electric Institute Publication VOL. 12B, No. 2 (1986), on pages 9 to 16. It should be noted that the arrangement of FIG. 17 is indicated by rewriting the construction of the conventional control circuit of the inverter described in the above-explained Japanese publication in a similar form to that of the present invention.
In FIG. 17, reference numeral 1 indicates a three-phase inverter main circuit, reference numeral 2 indicates a load such as a motor, reference numeral 4 represents a DC power supply, and symbols 10U, 10V; 10W indicate current sensors for detecting an inverter current. Also, reference numeral 801 shows a three-phase sine wave current command generating circuit, reference numeral 802 shows a current deviation vector detecting circuit, reference numeral 803 is a back electromotive force predicting circuit for predicting back electromotive forces VBOU, VBOV, VBOW which are produced across the load, reference numeral 804 shows a PWM pattern table circuit, and reference numerals 851U, 851V, 851W indicate adders/subtracters.
Also, in FIG. 17, reference numerals 21U, 21V, 21W show internal inductances of the load 2, and reference numerals 22U, 22V, 22W show internal-induced voltages of the load 2.
FIG. 18 is a diagram for representing the arrangement of the three-phase inverter main circuit 1.
As indicated in FIG. 18, this three-phase inverter main circuit 1 is arranged by, for instance, a full-bridge circuit by employing switching elements S1 to S6.
Next, operations of the conventional control circuit used in the power converting apparatus will now be explained with reference to drawings.
In FIG. 17, the control circuit is arranged as a current control loop for performing an instantaneous current control. The adders/subtracters 851U, 851V, 851W calculate current deviations xcex94iU, xcex94iV, xcex94iW between current command values IAU*, IAV*, IAW*, and inverter currents IAU, IAV, IAW detected by the current sensors 10U, 10V, 10W. The current command values correspond to outputs of the three-phase sine wave current command generating circuit 801, and should be supplied by the inverter. The back electromotive force predicting circuit 803 predicts the back electromotive forces VBOU, VBOV, VBOW produced across the load from the current deviations xcex94iU, xcex94iv, xcex94iW so as to acquire a back electromotive force vector VB, and then, detects which region selected from a region [I] to a region [VI] indicated in FIG. 19 this back electromotive force vector VB is present.
FIG. 19 is a diagram for showing the six regions [I] through [VI] which are segmented by 8 sorts of voltage vectors V0 to V7, which are outputted in response to conditions of the switching elements of the inverter 1.
The current deviation vector detecting circuit 802 obtains a current deviation vector xcex94I from the above-explained current deviations xcex94iU, xcex94iV, xcex94iW, and then, detects which region selected from regions (1) to (7) shown in FIG. 20 this current deviation vector xcex94I is present.
For the sake of convenient explanations, circled numerals which are shown in the respective drawings are described as (1), (2), (3) etc., in this specification.
A predetermined allowable range which is defined based upon precision of a current control is set with respect to the current deviation vector xcex94I and the region (7) indicates that this current deviation vector xcex94I is located in the allowable range. The regions (1) to (6), which are located at an outer circumference of this region (7), represent that the current deviation vector xcex94I is located outside the allowable range.
The PWM pattern table circuit 804 selects switching modes k0 to k7 from both the region of the back electromotive force vector VB and the region of the current deviation vector xcex94I in accordance with a table of FIG. 22. The PWM pattern table circuit 804 determines switching conditions of the six switching elements employed in the three-phase inverter 1 shown in FIG. 21 based upon these switching modes k0 to k7.
For instance, in such a case that the back electromotive force vector VB is present in the region [I], this PWM pattern table circuit 804 selects the switching mode k1 when the current deviation vector xcex94I is located in either the region (1) or the region (5). Also, this PWM pattern table circuit 804 selects the switching mode k3 when the current deviation vector xcex94I is located in either the region (2) or the region (3). Also, this PWM pattern table circuit 804 selects the switching mode k0 or k7 when the current deviation vector xcex94I is located in either the region (4) or the region (6). Also, the PWM pattern table circuit 804 selects a proper switching mode such that this switching mode is directly kept when the current deviation vector xcex94I is located in the region (7).
The three-phase inverter 1 turns ON/OFF the switching elements in response to the switching command of the PWM pattern table circuit 804 so as to control the inverter currents IAU, IAV, IAW.
Next, an explanation is made of how the current deviation vector xcex94I is transferred under the above-explained control operation.
For example, the following case will now be considered. That is, in FIG. 19, the back electromotive force vector VB is present at VL with the region [I]. Also, in FIG. 20, the current deviation vector xcex94I is present at xcex94Ia within the region (1).
From FIG. 22, the switching mode k1 is selected under this condition, and the current deviation vector xcex94I is moved along the direction of VL1 equal to a difference between VL and V1 shown in FIG. 19, and then, is entered from xcex94Ia of FIG. 20 into the region (7) within the allowable range.
However, in such a case that the current deviation vector xcex94I is located at xcex94Ib shown in FIG. 20, the switching mode k1 is similarly selected, whereas the current deviation vector xcex94I is not entered into the allowable range, but is once moved to the region (3). Subsequently, since the switching mode k3 is selected based upon both the region [I] and the region (3), the current deviation vector xcex94I is moved along the direction of VL3, and then is entered into the region (7) which is located in the allowable range.
In this case, if the switching mode k3 is selected at such a time instant when the current deviation vector xcex94I is present at xcex94Ib, then the current deviation vector xcex94I is moved along a dotted line of FIG. 20. As a result, it is most probably possible to enter the current deviation vector xcex94I into the region (7) in the allowable range by changing the switching mode one time.
The above-described conventional control circuit for the power converting apparatus has the following problem. That is, since there is such a possibility that the optimum output voltage vector used to enter the current deviation vector xcex94I into the allowable range cannot be selected only once, extra switching operations are carried out. As a result, the power losses of the switching elements are increased, and therefore, the efficiency of the power converting apparatus is lowered.
The present invention has been made to solve the above-explained problem, and therefore, has an object to provide a control circuit of a power converting apparatus capable of selecting the optimum output voltage vector, while reducing a total number of switching times.
According to a first aspect of the present invention, there is provided a control circuit of a power converting apparatus, characterized by comprising: current detecting means for detecting an output current of the power converting apparatus; three-phase current command generating means for generating a current command value; adding/subtracting means for calculating a current deviation between the current command value and the output current; voltage detecting means for detecting a voltage of a three-phase power supply which is connected via a reactor to the power converting apparatus and for acquiring a power supply voltage vector from the detected voltage of the three-phase power supply; and switching command generating means operated in such a manner that a current deviation vector is obtained from the current deviation, an allowable range region is set with respect to the current deviation vector, in the case that the resulting current deviation vector is not located within the allowable range region, a moving direction of the current deviation vector as to a plurality of output voltage vectors of the power converting apparatus is obtained based upon the resulting power supply voltage vector, and then, the switching command generating means outputs such an output voltage vector in which the moving direction of the current deviation vector among the resulting plural moving directions is directed to the allowable range region.
A control circuit of a power converting apparatus according to a second aspect of the present invention is characterized in that the switching command generating means selects such an output voltage vector in which the moving direction of the current deviation vector among the resulting plural moving directions is directed to the allowable range region, and a time duration required to cause the current deviation vector to penetrate the allowable range region is the longest time.
A control circuit of a power converting apparatus according to a third aspect of the present invention is characterized in that the power converting apparatus is a three-phase inverter constituted by a plurality of switching elements; and the switching command generating means selects an output voltage vector in which the moving direction of the current deviation vector among the resulting plural moving directions is directed to the allowable range region, and such a value obtained by multiplying the time duration required to cause the current deviation vector to penetrate the allowable range region by a weight coefficient is the longest value; the weight coefficient corresponding to a total switching time of the plural switching elements, which is required for changing the switching mode.
A control circuit of a power converting apparatus according to a fourth aspect of the present invention is characterized in that the switching command generating means calculates such an evaluation function, and selects the output voltage vector based upon the evaluation function, the evaluation function being calculated by multiplying a time duration required in that the current deviation vector passes through the allowable range region when a zero voltage vector is outputted by a weighing coefficient which is determined by a total switching time and is required for changing the switching mode.
A control circuit of a power converting apparatus according to a fifth aspect of the present invention is characterized in that the allowable range region is a hexagon.
A control circuit of a power converting apparatus according to a sixth aspect of the present invention is characterized in that the respective edges of the hexagon are arranged so as to be intersected at a right angle with respect to actual voltage vectors outputted from the power converting apparatus.
A control circuit of a power converting apparatus according to a seventh aspect of the present invention is characterized in that the respective edges of the hexagon are arranged so as to be intersected at an angle of 60 degrees with respect to actual voltage vectors outputted from the power converting apparatus.
A control circuit of a power converting apparatus according to an eighth aspect of the present invention is characterized in that when the resulting current deviation vector is not present in the allowable range region, the switching command generating means does not change a switching command to be outputted in such a case that a presently-outputted switching command causes the current deviation vector to be moved along the direction of the allowable range region.
A control circuit of a power converting apparatus according to a ninth aspect of the present invention is characterized in that the switching command generating means calculates such an evaluation function, and selects the switching mode based upon the evaluation function, the evaluation function being calculated by multiplying a time duration required in that the current deviation vector passes through the allowable range region when a zero voltage vector is outputted by a weighing coefficient which is determined by an unbalanced use of switching times with respect to each of the phases, and is required for changing the switching mode.
A control circuit of a power converting apparatus according to a tenth aspect of the present invention is characterized in that the switching command generating means acquires the moving direction of the current deviation vector based upon a change rate of a current command.